The Gram-positive opportunistic pathogen Staphylococcus aureus is the causative agent of minor skin infections to far more severe hospital and community-acquired diseases. New antibiotic therapies that target novel metabolic pathways are therefore urgently needed and sulfur metabolism represents one such validated antimicrobial and vaccine strategy. Staphylococcus aureus is characterized by a unique thiol metabolism and is strongly restricted in its ability to obtain inorganic sulfur to make cysteine, an essential amino acid. We have discovered a paralog of a copper-sensing transcriptional repressor CsoR (copper-sensitive operon repressor) in S. aureus strain Newman that we denote CstR, for CsoR-like sulfurtransferase repressor. CstR regulates the expression of a novel operon, cst, which encodes a putative sulfite/sulfonate effluxer (TauE) as well as two multidomain proteins that harbor canonical thiosulfate sulfurtransferase or rhodanese domains (CstA, CstB) whose biological functions are unknown. Our specific objectives are to (1) Elucidate the mechanistic basis of CstR-mediated repression and derepression of cst operon transcription; (2) Elucidate the structure and mechanism of the multidomain thiosulfate sulfurtransferase CstA, which we hypothesize harbors three sulfur relay modules that collectively mediate vectorial transfer of cysteine persulfides from donor to acceptor. NMR structural studies and a novel 34S-32SO32- pulse-32S-32SO32- chase experiment are proposed; (3) Identify cellular proteins that transiently carry 34S-persulfide groups originating with extracellular sodium 34S- 32SO3 thiosulfate using a bottom-up proteomics approach; and 4) Begin to elucidate the enzymatic activities of CstR-regulated gene products CstB, a putative rhodanese-containing sulfur dioxygenase and SQR, a putative sulfide:quinone oxidoreductase. This project will provide new insights into the mechanisms of sulfur metabolism and resistance to sulfur metabolite toxicity in an important human pathogen. These studies will significantly extend the emerging paradigm of a cysteine persulfide-based sulfur shuttling system in other bacteria to a new metabolic process as a potential novel antimicrobial intervention strategy.